Since you state that your application is complex and setting a test project would be problematic, I have 2 suggestions for you:
1. Set ShowContentDuringLoad = true
for the RadWindowManager.
For large pages such as yours this property can make a visible difference.
2. In case this does not make a notable difference, please make the same test with a regular IFRAME - and see whether it behaves the same.
E.g. instead of RadWindowManager, add an IFRAME to your page.
<button onclick="document.getElementById('testIframe').src='SlowLoadingPage.aspx';return false;">Click me</button>
Of course, the best option would be (as already mentioned) if someone could send us a sample project that demonstrates the difference in loading time in RadWindow and out of RadWindow - so that we can test properly on our end.
the Telerik team
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